Using the address in Postman

In Postman, the proxy is under Settings, on the Proxy tab. Turn on the custom proxy, enter the IP and port 20001, and switch authentication on with the username and password.

Checking

Make a request to a service that returns your IP. If the address IP comes back, it is in effect. If your machine IP comes back, Postman is ignoring the configuration, which happens when the system proxy is ticked instead of the custom one.

This configuration applies to every request. There is no per-request setting.